Confirming possible soakaway issue

I've been recently informed by my immediate neighbour that he thinks our soakaway is damaged (or incorrectly constructed) and I'm trying to see how I can confirm this. If I can confirm there is an issue, and I can locate the soakaway I'll happily get it repaired or replaced.

The background is that we live in a 15 year old house on a steep hill and the two houses share a boundary. About six months ago the neighbour removed some soft landscaping and extended his drive with block paving.

The area can be seen in the following image. Our house is on the right. Previously soil height went up to the bottom of our cribbing (as indicated by the pale patch on the concrete footing) and sloped towards the tarmac on the left. It was covered in established shrubs, bushes and three small trees.

Now that area has been changed from soft landscaping to hard landscaping it has been noticed that in heavy downpours a volume of water appears to flow from the gravel at the top of the block paving, washing the sand that has been used to bed it in down the road.

To date the water is flowing from the gravel in two places; from above the BT box and approximatey 1.5 metres towards the houses. Each area of flow is between 50cm and 1m wide. It does not appear to be flowing over the concrete footing or the soil onto the gravel.

I do have a video, but unfortunately I'm struggling to upload it.

This flow of water is understandably causing the neighbour concern, and he's asked me to get it rectified as soon as possible. He can then reinstate the sand and repair any damage to the sand bedding under the blocks.

I've spoken to one of the builders involved with the site 15 years ago and he tells me that from his recollection we have "open tank soakaways" and that he is not aware of issue reason they would start to fail.

Therefore, my questions to the DIYnot.com collective wisedom are how can I go about confirming that the soakaway is at fault, and how can I confirm its location?

I've lifted the closest rainwater inspection cover and found no water backing up. I've also put a hose down the closest downpipe to the area in order to see if I can get water to appear in the same location and so far I'm failing miserably. It stays dry.

Any advice would be greatly received. I've been searching the web in order to find a competent professional that may be able to help, but at present I've drawn a blank.

Why when there is a grate and a man hole to the extreme right in the photo, did they use a soak-away?

I'm no expert on soak-aways, but assume in is located just to the left of the house in the photo(?), where that gravel seems to have been washed out. I'm surprised it is so close to the side of your land behind the fence and so close to that path/road on such a slope too. I cannot see a BT box, unless you meant the lighter coloured manhole cover in the block paving?

I assume the 'open tank soakaways' means it is just like a crate, buried in the soil acting as a tank for it to fill then for it to gradually leak away into the surrounding soil, which if that is what it is and is built into sloping land, it will drain too rapidly down that slope. Was the block paved area sometimes quite muddy before the paving was laid?

Yes, I'll take a look and try this, however the grate and manhole cover to the right are on the road, not on our property. That feeds into a soakaway for the road and there is no suggestion that this is, or could, cause the water flow.

Our house is the one on the right and our neighbour tells me that the soakaway is either under the grass or our drive, and it is failing, and then the water is flowing over his blocks and washing the sand out.

Yes, I'll take a look and try this, however the grate and manhole cover to the right are on the road, not on our property. That feeds into a soakaway for the road and there is no suggestion that this is, or could, cause the water flow.

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Well it was a long shot, bu tits not unknown for road storm drains to flood gardens -broken pipe or whatever.

Our house is the one on the right and our neighbour tells me that the soakaway is either under the grass or our drive, and it is failing, and then the water is flowing over his blocks and washing the sand out.

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its possible, the problem is Im not sure a hosepipe would be able to fill a soakaway quickly enough to test this theory.

generally soakaways silt up, then they dont rain fast enough and fill up until the water shows on the surface -easy to see on a lawn. Old soakaways are usually just a pile of rubble. newer ones can be hollow tanks or crates.

I suppose you could test all the downpipes on your house to see if you can get water to come out.

if its a soakaway filling up, its gonna be a real pain digging it up and sorting out -especially as you dont know if or where it is.

if i read your first post correctly, the ground level used to be higher abutting your property until the neighbour put in his hard landscaping.

if that's correct, then is suspect that your soakaway is no longer set in deep enough ground on the side adjacent to your neighbour to allow the water to soak away and disperse. Consequently, when the water level in the soakaway is high the water level is close to the surface and the water is coming through a relatively thin layer of top soil and flowing out down the slope as opposed to soaking away into a mass of soil.

If i'm correct then i would suspect the only way of resolving this is to reduce the flow of water into the soakaway by redirecting some downpipes into your foul drain (probably not legal) or by digging the soakaway out and setting it deeper into the ground.

Lower makes some good points above regarding the soakaway which tally with NHBC Standards guidance on positioning of soakaways on sloping sites.
As others have said surface water run off over the sloping ground is a different matter to water

percolating into the ground from the soakaway and is always going to flow down over a non permeable hard surface.​

Is the ground suitable for soakaways anyway , percolation tests should have been done
Soakaways should be at least 5m from the foundations of a house or a road according to building regulations.
Removing soft landscaping and replacing with hard surfaces doesn't help of course with allowing rainwater to disperse.

If i'm correct then i would suspect the only way of resolving this is to reduce the flow of water into the soakaway by redirecting some downpipes into your foul drain (probably not legal)

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- don't discharge rainwater into a foul drain. Only connect rainwater pipes to a surface water drain or into a combined drainage system when rwp's should have trapped gullies.
p.s but it won't be a combined system of course or they wouldn't have used soakaways